BUT WHILE NASDAQ'S REVENUE has been flat over the past three years, its earnings have grown at a 14% clip, and its profit margin trumps the NYSE's. Last quarter, U.S. stock trading accounted for just 7% of revenue. Add European stocks and global derivatives, and trading still drives just 29% of its haul. The bulk of revenue comes from recurring operations like providing market data (20%), listings and issuer services (24%), technology (11.5%), and access and broker services (15.5%). This nontrading revenue will swell further, to 75%, once Nasdaq closes its purchase of Thomson Reuters' investor-relations business. "There was a time when we were properly identified as a stock exchange," Nasdaq CEO Robert Greifeld tells Barron's. "But now, we're stuck with that typecast."

Nasdaq's steady income and an 11% free cash-flow yield makes buyout firms' mouths water. Ironically, there isn't much for new owners to squeeze. "Nasdaq doesn't need a lot of restructuring," says Sandler O'Neill analyst Rich Repetto. "It's also run efficiently, so there aren't a lot of costs to be taken out."

A privately owned market for public stock trading isn't in itself a problem; many exchanges once were private. But ownership was diffuse and spread among many seat holders, and it remains to be seen if regulators will permit a key securities market to be controlled by one buyout firm.

Nasdaq has a market value of $5 billion, and BMO Capital Markets pegs its takeout value at $6.45 billion. Another sum-of-the-parts analysis goes as high as $9.17 billion, assuming buyers would pay peer-average multiples for each unit (like 18 times 2012 earnings for listing services, or 23.7 times for market technology). That translates to $38 to $54 a share. But buyout firms can't easily wring profit at that range's high end. Nasdaq also has $2 billion of debt, which limits the leverage that firms can pile on.

A strategic buyer might do better, because merging overlapping trading engines, data centers, and back offices can extract big savings. But the IntercontinentalExchange has picked NYSE, and Deutsche Bourse and CME "remain committed to derivatives and post-trade processing," notes BMO analyst Jillian Miller. Ambitious Asian exchanges with rich valuations are keen, but some have state ties that won't easily pass U.S. regulatory muster.

But even without a buyout, analysts say Nasdaq is worth $28 to $30 a share. Nasdaq and other U.S. options exchanges also own the Options Clearing Corp, which clears and guarantees all U.S. options, although Greifeld says there's no plan to monetize this lucrative asset. And while Nasdaq's units are complementary, Greifeld is taking steps to show "they're, in fact, separable"— streamlining data and indexes, or consolidating the technology platform—to emphasize each business. "If we perform the way we plan, our [valuation] multiple should take care of itself," he says.

Sean Bonner, who manages the Carne Large Cap Value Fund, likens exchanges to tollbooths for transactions and trading data. Technology has made the highway bigger and faster, but the asset base—servers, software, a few buildings—are small, compared to their profitability. "It's literally a bucket nailed to a post on the side of the road where people throw money," he maintains.

A TARGET COMPANY HOLDING OUT for better offers just may be a sign that our economy is strengthening. Will a strengthening economy swell trading traffic? Big mergers are ticking up (see Preview). New listings have increased as stock prices rise and volatility subsides, and Dealogic says U.S. IPOs have raised $6.03 billion so far this year—35% better than the average since 2001.

With the 10-year Treasury yield just above 2%, the crop of Standard & Poor's 500 stocks sporting higher dividend yields last week reached 228, or 46%—well above long-term norms near 5% or 6%. "That number won't stay high for long," says Owen Fitzpatrick, head of U.S. equity strategy at Deutsche Asset & Wealth Management. "Over time, either Treasury yields start climbing, or people buy enough stocks to drive their yields down."